I’d like to find bits/burs for my flexshaft in various sizes that cut
like a machinist’s end mill (equivalent to drill bit sizes ranging
from 77-50). In other words, they look like drill bits, but are end
cutting and leave the bottom of the hole flatter than a drilled hole
and the sides of the hole perpendicular to the bottom. Do such
bits/burs exist in “jewelry sizes?”

who has mini mills for $7 each, in a variety of sizes. So it could
be that my head is out of date, or this guy’s wildly cheap. Dunno
which.

Drawback #2 is that they aren’t intended for hand use. They’rebrittle, and they are very, very agressive in comparison to a
standard burr.

If you use them in a miniature drill press, they will indeed give
you exactly the flat bottomed hole you’re looking for. The support of
the drill press keeps them from digging in. If you try to freehand
them in a flex-shaft, they’ll likely dig in and snap almost
instantly. (I’d try a very light cut, and the highest speed your
machine can provide, to keep the flutes from digging in.) (andABSOLUTELY shalt thou wear safety glasses! It’s not a question of
odds: there will be high speed fragments.)

I’m at sixes and sevens about whether to get 2 flute mills or 4. 2
flutes clear soft material (like wax & silver) better, but are more
likely to grab and snap when used by hand. 4 flute mills won’t grabquite as easily, but it’s easier to jam them full of crap, and
break them that way. The more I think about it, the more I think 4
flute mills are probably safer if you can find them.

The other fun thing is the chips: depending on the end mill, and how
you use it, the chips coming out can frequently look like tiny
little silver needles. Not only do they look like microscopic
needles, they act like invisible little needles, jamming themselves
into your skin in places where you can’t find them without a
microscope. But you can sure feel them…

these are called flat drills. They are exist in watchmaker sizes,
which are too small for jewellery. They are not difficult to make if
you want to spent some time. Another option is to grind bottom of
round bur flat. Good way to give second life to old burs.

Flat bottom rotary files are readily available to purchase from
places like MSC in straight cut and cross cut, and fine or coarse.
If you need a special size they can be custom made in quantities and
will last with proper use. You could use a smaller size and make a
jig to get to the exact size you want.

Take a blunt or broken drillbit of desired size and grind or file
with diamond file, squaring the end off. Pre drill work to desired
depth with a new drillbit then change to prepared drillbit and
carefully drill out the tiny amount necessary. Hope this helps.